A Brønsted-Lowry base is defined as a substance that can accept protons (H+) in a chemical reaction. This concept is key in understanding acid-base chemistry, as it highlights the role of bases not just as neutralizers of acids, but as active participants that interact with protons. The Brønsted-Lowry theory also emphasizes the relationship between acids and bases through conjugate pairs, where a base transforms into its conjugate acid upon proton acceptance.